/*
   @Copyright:LibreOJ
   @Author:   tjyemail
   @Problem:  https://loj.ac/problem/7
   @Language: C++
   @Datetime: 20-07-30 13:07
   */
// scanf("%lld"); time 917ms
#include <bits/stdc++.h>
using namespace std;

int main()
{
	unsigned long long ans = 0, a;
	for(int n=3000000; n--;){
		scanf("%lld", &a);
		ans ^= a;
	}
	printf("%lld\n", ans);
	return 0;
}
